<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  #app{
    width: 100px;
    height: 100px;
    /* transform 会影响 position: fixed; 效果 */
    transform: scale(1);  
  }
  .fixed{
    position: fixed;
    background: blue;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
  }
</style>
<body>
  <div id="app">
    <div class="fixed"></dix>
  </div>
  
</body>
</html>